1. Field of the Invention
The present invention relates generally to holographic storage systems.
2. Related Art
Developers of information storage devices and methods continue to seek increased storage capacity. As part of this development, page-wise memory systems, in particular holographic systems, have been suggested as alternatives to conventional memory devices. Holographic systems typically involve the storage and readout of entire pages of information, these pages consisting of arrayed patterns representing information. In general, a holographic system stores, in three dimensions, holographic representations of the pages as patterns of varying refractive index and/or absorption imprinted into a storage medium.
Holographic systems are characterized by their high density storage potential and the potential speed at which the stored information is randomly accessed and retrieved. In fact, because information is typically manipulated, i.e., stored and retrieved, on a page-by-page basis, the speed of storage and retrieval compares favorably to conventional magnetic disk or compact disk storage systems. A significant advantage of holographic systems, however, is storage capacity. It is possible for each page stored as a holographic image to contain thousands or even millions of elements. Theoretically, it is believed that at the present time, up to 1014 bits of information are storable in approximately 1.0 cm3 of holographic storage medium.
Multiplexing holograms means to store multiple holograms in the same volume or nearly the same volume. Typically, this is done by varying an angle, wavelength, phase code, or some other system parameter in the recording and readout setup. Many of these methods rely on a holographic phenomenon known as the Bragg effect in order to separate the holograms even though they are physically located within the same volume of media. Other multiplexing methods such as shift and, to some extent correlation, use the Bragg effect and relative motion of the media and input laser beams to overlap multiple holograms in the same volume of the media.
Using holography to store data has been well known for the last 30 years. The idea of increasing system capacity by combining spatial multiplexing (recording holograms in multiple locations but not significantly in the same volume of media) along with some other multiplexing technique that overlaps holograms within the same location has been well known for over 15 years. These multiplexing techniques have been used for distributing holograms on a disk, card, cube, or tape. Several patents and papers describe systems that take advantage of the combination of multiplexing techniques. For example, U.S. Pat. No. 6,798,547 to Wilson et al.; U.S. Pat. No. 6,721,076 to King et al.; U.S. Pat. No. 6,697,180 to Wilson et al. and U.S. Pat. No. 6,614,566 to Curtis et al. and U.S. Published Patent Application No. 2004-0179251 to Anderson, et al. describe multiplexing holographic storage techniques and the entire contents and disclosures of the above patents and patent application are hereby incorporated by reference. The paper Tao et al., “Spatioangular Multiplexed Storage of 750 Holograms in a FeLiNbO3 Crystal” in Optic Letters, Vol. 18, No. 11, 912-1014 (1993) also describes multiplexing storage techniques and the entire contents and disclosure of this paper is hereby incorporated by reference.